CARACAS - Venezuela will postpone the inauguration of President Hugo Chavez for a new term due to health problems, the government said on Tuesday, another sign the socialist leader's cancer may be bringing an end to his 14 years in power.

The 58-year-old former soldier who has dominated the South American OPEC nation since 1999 has not been heard from since surgery on 11 December in Cuba - his fourth operation since he was diagnosed with an undisclosed type of cancer in June 2011.

The announcement outraged opposition leaders who insist that Chavez must be sworn in before the National Assembly on 10 January as laid out in the constitution, or temporarily step aside and leave an ally in power.

"The commander president wants us to inform that, based on his medical team's recommendations, the post-operative recovery should extend past 10 January," said Vice President Nicolas Maduro, Chavez's chosen successor, in a letter read to the legislature.

"As a result, he will not be able to be present at the National Assembly on that date."

The letter said authorities would seek another date for the inauguration ceremony but did not say when it would take place or give a time frame for Chavez's return from Havana.

Rather than being sworn in by the legislature, he would take his oath at a later date before the Supreme Court, the letter said, as allowed by the constitution.

Government leaders insist Chavez is completely fulfilling his duties as head of state, even though official medical bulletins say he has a severe pulmonary infection and has had trouble breathing.

The government has called for a massive rally outside the presidential palace on Thursday, and allied presidents including Uruguay's Jose Mujica and Bolivia's Evo Morales have confirmed they will visit Venezuela this week despite Chavez's absence.

Argentine President Cristina Fernandez has announced plans to visit Chavez in Havana on Friday.

But the unprecedented silence by the president - famous for regularly speaking for hours in meandering broadcasts - has left many convinced he could be in his last days.

His resignation or death would upend politics in the oil-rich nation, where he enjoys a deity-like status among poor supporters thankful for his social largesse.

His critics call him a fledgling dictator who has squandered billions of dollars from crude sales while dashing the independence of state institutions.